Polymer network liquid crystal (PNLC) is liquid crystal that has a polymer fiber structure (polymer microphase-separated structure; so-called polymer network) therein. When no voltage is applied, the liquid crystal molecules in the polymer network liquid crystal are randomly orientated along the polymer fibers, causing the refractive index of the liquid crystal to differ from the refractive index of the polymers, and therefore, light is scattered. This results in an opaque appearance of the polymer network liquid crystal. On the other hand, when a voltage is applied, the liquid crystal molecules are aligned. Because the refractive index of the liquid crystal and the refractive index of the polymers are matched at this time, the scattering of light can be suppressed and the liquid crystal becomes transparent.
Various liquid crystal display panels that employ the polymer network liquid crystal (may also be referred to as “PNLC display panels” below) have been proposed. In addition to an application to reflective display panels (such as electronic papers, for example), the PNLC display panels can also be used for transmissive liquid crystal display panels as described in Patent Document 1, for example.
A typical manufacturing method of a PNLC display panel includes a step of filling a gap between two display panel substrates with a liquid crystal material having monomers mixed therein, and a step of polymerizing the monomers to form polymer network. As the method of polymerizing the monomers, a method of radiating light energy such as ultraviolet light is employed, for example.
In the display panel substrates, prescribed elements such as wiring lines are formed. When the PNLC display panel is an active matrix type liquid crystal display panel, for example, a TFT array substrate and a color filter are used as the display panel substrates. In the TFT array substrate, elements such as thin film transistors (TFTs), gate wiring lines (may also be referred to as “scan lines”), source wiring lines (may also be referred to as “data lines”), and auxiliary capacitance wiring lines (may also be referred to as “Cs wiring lines,” “holding capacitance wiring lines,” or the like) are formed. In the color filter, elements such as a black matrix (BM) and colored patterns of prescribed colors are formed.
For this reason, in the step of polymerizing the monomers, light energy may be blocked by elements having a light-shielding property, and part of the liquid crystal material having monomers mixed therein may not be irradiated with the light energy. As a result, the part that was not irradiated with the light energy become a monomer unreacted region (region in which monomers are not polymerized). The light transmittance and the refractive index may differ between the monomer unreacted region and portions that have been polymerized. Therefore, presence of the monomer unreacted region may cause “seeping of light” inside the regions or at interfaces thereof, resulting in the display non-uniformity. Also, because the monomer unreacted region has a different physical property from that of the polymerized portions, if an external force is applied to the PNLC display panel that has the monomer unreacted region, the stress may not be absorbed in the monomer unreacted region, causing air bubbles (substantially vacuum spaces; may also be referred to as “vacuum bubbles” below) to be formed. The light transmittance and the refractive index of the vacuum bubbles are different from those of other portions, and therefore, “seeping of light” may occur in the vacuum bubbles, resulting in the display non-uniformity.
Therefore, it is preferable that the monomer unreacted region be prevented from being formed in the step of polymerizing the monomers, and in order to do so, it is necessary to radiate the light energy to the entire liquid crystal material having the monomers mixed therein (so as not to have a region where the light energy is not radiated).
Some of the elements that are formed in the display panel substrates need to be protected from the light energy radiation. For example, when the thin film transistors that are formed in the TFT array substrate are irradiated with the light energy, the electrical characteristics thereof change. Therefore, it is necessary to block the light energy by using a black matrix formed in the color filter so as to prevent the thin film transistors from being irradiated. This means that the monomers present in regions that are blocked by the black matrix of the color filter would not be irradiated with the light energy, and such regions become the monomer unreacted regions.
As described above, in some cases, it is difficult to have the entire liquid crystal material, which has monomers mixed therein, irradiated with the light energy in the step of polymerizing the monomers.
To solve this problem, in the PNLC display panel, a configuration in which thin film transistors are covered by light-shielding films, and a dielectric multi-layer film that allows ultraviolet light to pass through while blocking visible light is formed in positions that face the thin film transistors has been proposed (see Patent Document 2). With this configuration, it is possible to irradiate the monomers with ultraviolet light that has passed through the dielectric multi-layer film. However, because this configuration requires a process of forming the dielectric multi-layer film, the number of process steps in the process of manufacturing the display panel is increased. This leads to an increase in the equipment cost and an increase in the manufacturing cost.